The present invention relates to a semiconductor device fabrication method, more specifically, a semiconductor device fabrication method in which a film-to-be-polished is polished.
Conventionally, as a technique for forming device isolation regions for defining device regions, LOCOS (LOcal Oxidation of Silicon) has been widely known.
However, when device isolation regions are formed by LOCOS, bird's beaks decrease the device regions. Reducing the oxidation amount for forming the device isolation regions can make the bird's beaks smaller, but when the oxidation amount is small, the device isolation function is not sufficient. When the device isolation regions are formed by LOCOS, large steps are formed on the substrate surface. Thus, the technique for forming device isolation regions by LOCOS has found further micronization and higher integration difficult.
As a technique which takes over LOCOS, STI (Shallow Trench Isolation) is noted. The technique for forming device isolation regions by STI will be explained with reference to FIGS. 47A to 47C. FIGS. 47A to 47C are sectional views of a semiconductor device in the steps of the method for fabricating the semiconductor device, which show the method.
First, as shown in FIG. 47A, a silicon oxide film 212 and a silicon nitride film 214 are sequentially formed on a semiconductor substrate 210.
Next, the silicon nitride film 214 and the silicon oxide film 212 are patterned by photolithography. Thus, openings 216 are formed in the silicon nitride film 214 and the silicon oxide film 212 down to the semiconductor substrate 210.
Next, the semiconductor substrate 210 is anisotropically etched with the silicon nitride film 214 as the mask, which has the openings 216 formed in. Thus, trenches 218 are formed in the semiconductor substrate 210.
Then, as shown in FIG. 47B, a silicon oxide film 220 is formed in the trenches 218 and on the silicon nitride film 214. The silicon oxide film 220 is to be a film-to-be-polished.
Next, as shown in FIG. 47C, the surface of the film-to-be-polished 220 is polished by CMP (Chemical Mechanical Polishing) until the surface of the silicon nitride film 214 is exposed. The silicon nitride film 214 functions as the stopper for polishing the film-to-be-polished 220. As the polishing slurry is used a polishing slurry containing, e.g., abrasive grains of silica and an additive of KOH. Thus, a device isolation regions 221 of the silicon oxide film 220 are buried in the trenches 218. Device regions 222 are defined by the device isolation regions 221.
Then, the silicon nitride film 214 and the silicon oxide film 212 are etched off. Then, transistors (not shown) are formed in the device regions 222. Thus, a semiconductor device is fabricated.
Forming the device isolation regions 221 by STI generates no bird's beaks, as does forming device isolation regions by LOCOS, and accordingly, the decrease of the device regions 222 can be prevented. The depth of the trenches 218 are set large, whereby the effective inter-device distance can be larger, and high device isolating function can be obtained.
However, the semiconductor device fabrication method using the above-described polishing slurry, i.e., the polishing slurry containing abrasive grains of silica and an additive of KOH, does not have high polishing rate and cannot always provide good planarity.
As a polishing slurry whose polishing rate is high and can provide good planarity is proposed, a polishing slurry containing abrasive grains and an additive of a surfactant. In the proposed polishing slurry, the abrasive grains are, e.g., cerium oxide (CeO2). The additive is, e.g., poly(ammonium acrylate).
FIGS. 48A to 48C are conceptual views of the mechanism for polishing a film-to-be-polished with the proposed polishing slurry. FIGS. 49A and 49B are sectional views of the film-to-be-polished polished by the proposed semiconductor device fabrication method.
FIGS. 48A and 49B show the state of the film-to-be-polished before the polish.
As shown in FIGS. 48A and 49A, concavities and convexities are present in the surface of the film-to-be-polished 220. On the surface of the film-to-be-polished 220 having the concavities and convexities, the additive 224 of a surfactant adheres to the concavities to thereby hinder the polish of the film-to-be-polished 220 in the concavities. On the other hand, high pressures are applied to the convexities to thereby release the additives 224 of a surfactant, and the polish of the film-to-be-polished 220 is not hindered. Thus, the convexities on the surface of the film-to-be-polished 220 is selectively polished by the abrasive grains 226. The surface of the film-to-be-polished 220 is thus planarized. The polish for planarizing the surface of the film to-be-polished is called the main polish.
FIG. 48B shows the surface of the film-to-be-polished which have been planarized.
On the planarized surface of the film-to-be-polished 220, the additive 224 of a surfactant stays on the entire surface of the film-to-be-polished, and hinders the film-to-be-polished 220 from being polished. The polishing rate becomes extremely low. Resultantly, as shown n FIG. 49B, the film-to-be-polished 220 remains on the stopper film 214.
Here, it is proposed to set the film thickness of the film-to-be-polished 220 to make the height of the surface of the concavities in the film-to-be-polished 220 substantially equal to the height of the surface of the stopper film 214. However, generally, the film thickness of the film-to-be-polished 220 varies by about ±30 nm from a design value. When the film-to-be-polished 220 is formed thicker than a design value, the film-to-be-polished 220 remains on the stopper film 214.
The film-to-be-polished 220 remaining on the stopper film 214 prevents the stopper film 214 and the silicon oxide film 212 from being etched off, and the film-to-be-polished 220 on the stopper film 214 must be removed by another method.
As a method for removing the film-to-be-polished remaining on the stopper film 214 is proposed to further polish the film-to-be-polished 220 with the feed of the polishing slurry being stopped and with deionized water being fed.
FIG. 48C shows the state where with the feed of polishing slurry onto the polishing pad being stopped and with water being fed onto the polishing pad, the film-to-be-polished is further polished, i.e., the state of the finishing polish.
When the finishing polish is started, the polishing slurry used in the main polish remains between the silicon oxide film 220, which is the film-to-be-polished, and the polishing pad 228. The additive 224 contained in the polishing slurry is water soluble, and is removed in a short period of time when the deionized water is fed. The abrasive grains 226 contained in the polishing slurry, however, are not water soluble and cannot be easily removed, and remain between the film-to-be-polished 220 and the polishing pad 228. The additive 224 has contributed to, as described above, decreasing the polishing rate of the film-to-be-polished 220 when the surface of the film-to-be-polished 220 is planarized. The additive 224 having such function is removed for a short period of time, while the abrasive grains 226 contributing to the polish remain between the film-to-be-polished 220 and the polishing pad 228, whereby the remaining abrasive grains 226 can further polish the film-to-be-polished 220.
Such finishing polish is performed for a prescribed period of time, whereby the film-to-be-polished 220 remaining on the stopper film 214 can be removed from surface of the stopper film 214.
Thus, the polish of the film-to-be-polished 220 is completed.
After the polish of the film-to-be-polished 220 has been completed, the inspection as to whether or not the polish of the film-to-be-polished 220 is normal.
FIGS. 50A and 50B are a plan view and a sectional view of an inspection pattern of the stopper film, which is formed in a scribe line. FIG. 50A is the plan view. FIG. 50B is the sectional view along the line A-A′ in FIG. 50A.
FIGS. 51A and 51B are a plan view and a sectional view of an inspection pattern of a buried insulation film, which is formed in a scribe line. FIG. 51A is the plan view. FIG. 51B is the sectional view along the line A-A′ in FIG. 51A.
Specifically, it is inspected that the film-to-be-polished 220 is not left on the inspection pattern 232a formed on the scribe line 230. It is inspected whether or not the thickness of the buried insulation film 220 buried in an inspection trench 218a is within a prescribed inspection specified range. Based on the inspection result, it is judged that the polish of the film-to-be-polished 220 has been normal, the next step follows. When the polish of the film-to-be-polished 220 is not normal, the semiconductor device is treated as a defective.
Following references disclose the background art of the present invention.
[Patent Reference 1]
Specification of Japanese Patent Application Unexamined Publication No. 2001-9702
[Patent Reference 2]
Specification of Japanese Patent Application Unexamined Publication No. 2001-85373
[Patent Reference 3]
Specification of Japanese Patent Application Unexamined Publication No. 2001-338902
[Patent Reference 4]
Specification of Japanese Patent Application Unexamined Publication No. 2002-83787
[Patent Reference 5]
Specification of Japanese Patent Application Unexamined Publication No. Hei 11-104955
[Patent Reference 6]
Specification of Japanese Patent Application Unexamined Publication No. 2000-248263
However, in the proposed semiconductor device fabrication method, the depth of concavities called dishings 223 formed in the surface of the buried oxide film 220 often largely vary (see FIG. 52). FIG. 52 is a sectional view of the dishing formed in the surface of the buried oxide film.
In the proposed semiconductor device fabrication method, deep dishings 223 are often formed in the surface of the buried oxide film 220, or the film-to-be-polished 220 often remains on the device region 222.
When the film-to-be-polished 220 is polished with the proposed polishing slurry, the depth of the dishings 223 largely vary depending on the area of the trenches 218, 218a. FIG. 53 is a graph of the film thickness of the buried oxide film buried in the trenches of 40 μm×40 μm and the film thickness of the buried oxide film buried in the trenches of 100 μm×100 μm, which compare the film thicknesses. As seen in FIG. 53, when the area of the trenches is large, the dishing 223 formed in the buried oxide film 220 is deep, and the film thickness of the buried oxide film 220 is thin. Accordingly, when the film-to-be-polished 220 is polished with the proposed polishing slurry, it is difficult to inspect accurately whether or not the polish of the film-to-be-polished 220 is normal.